Pierre and Marie Curie, a husband and wife team of physicists, discovered the spontaneous emission of particles from certain elements. They called this phenomenon "radioactivity." Together they won three Nobel prizes, and the element curium was named in their honor.

Radioactivity
is the tendency of certain atoms to decay into lighter atoms, a process that emits energy. Radioactivity also provides a way to find the absolute age of a rock. First, we need to know about radioactive decay.

Some isotopes are radioactive;
radioactive isotopes
are unstable and spontaneously change by gaining or losing particles. Two types of radioactive decay are relevant to dating Earth materials (
Table
below
):

One neutron decays to form a proton and an electron. The electron is emitted.

The radioactive decay of a
parent isotope
(the original element) leads to the formation of stable
daughter product
, also known as daughter isotope. As time passes, the number of parent isotopes decreases and the number of daughter isotopes increases (
Figure
below
).

Half-Lives

Radioactive materials decay at known rates, measured as a unit called
half-life
. The half-life of a radioactive substance is the amount of time it takes for half of the parent atoms to decay. This is how the material decays over time (see
Table
below
).

Radioactive Decay

No. of half lives passed

Percent parent remaining

Percent daughter produced

0

100

0

1

50

50

2

25

75

3

12.5

87.5

4

6.25

93.75

5

3.125

96.875

6

1.563

98.437

7

0.781

99.219

8

0.391

99.609

Pretend you find a rock with 3.125% parent atoms and 96.875% daughter atoms. How many half lives have passed? If the half-life of the parent isotope is 1 year, then how old is the rock? The decay of radioactive materials can be shown with a graph (
Figure
below
).

Decay of an imaginary radioactive substance with a half-life of one year.

Notice how it doesn’t take too many half lives before there is very little parent remaining and most of the isotopes are daughter isotopes. This limits how many half lives can pass before a radioactive element is no longer useful for dating materials. Fortunately, different isotopes have very different half lives.